Cycling experiments on redox characteristics of iron-rich coal ash under fluidized bed conditions

Oxygen carriers can establish reaction paths between fuels and oxidants and have been widely used in chemical looping systems and oxygen carrier aided combustion. Among various oxygen carriers, iron-rich coal ash with abundant reserves and good physical–chemical characteristics are ignored in present studies. This work aims to determine the redox activity of iron-rich Indonesian lignite (IN) ash via a cycling combustion experiment of biogas including H₂, CH₄, and CO in a bubbling fluidized bed reactor at 700℃, 800℃, and 900℃. Two types of Chinese iron-lean coal ash (Jiaocheng and Shuozhou coal ash) and two typical oxygen carriers (steel slag and ilmenite) were also tested for comparison. Results show that IN ash exhibits significant adsorption capacities and conversion rates for combustible gases (>0.028 mol/g and >17%) and oxygen (>0.02 mol/g and ≥68%), which are generally the highest at 800℃ (0.056 mol/g and 35.0% for combustible gases, 0.024 mol/g and 72.5% for oxygen) due to higher reaction activity and a lower degree of sintering and agglomeration. While these indexes are higher than those of iron-lean coal ash and comparable to those of steel slag and ilmenite, IN ash does not considerably promote partial oxidation of methane or carbon deposition, and its composition remains relatively stable through long-term experiments. The excellent redox activity of Indonesian lignite ash is mainly attributed to its high iron content and good pore characteristics. This work reveals the great potential of iron-rich coal ash to be applied as an oxygen carrier in oxygen-carrying processes and relevant industrial systems.